Continuing efforts are being made to improve the performance of field effect transistors (FETs) for microwave applications. Candidates for future high speed power transistors include metal semiconductor field effect transistors (MESFETS) comprising III-V compound semiconductor materials, e.g., GaAs, AlGaAs or InGaAs, and having double recessed gates to minimize the effects of surface charge.
High electron mobility transistors (HEMTs) are among the fastest microwave power transistors. Low noise performance at high frequencies has been demonstrated with these devices at low temperatures, e.g., 77 K. as well as at room temperature. High transconductance of the HEMT design has been attributed to the relatively thin (less than 100 .ANG.) conduction channel layer, commonly referred to as a two dimensional electron gas (2DEG), which results from a conduction band discontinuity at the interface of a selectively doped heterostructure. For example, in a n-AlGaAs/GaAs heterostructure silicon doped Al.sub.X Ga.sub.(1-X) As (X=0.3) is deposited by molecular beam epitaxy over an undoped layer of GaAs. Because the 2DEG layer is confined to a potential well in the undoped GaAs layer, high electron mobilities are observed.
In order to improve low temperature HEMT performance and avoid formation of DX centers in the Al.sub.X Ga.sub.(1-X) As layer the mole fraction of AL must be reduced to less than 18 percent. Because this effectively limits the conduction band gap between the Al.sub.X Ga.sub.(1-X) As layer and the GaAs layer, InGaAs, having a lower conduction band than GaAs, is chosen as the 2DEG channel material. This allows for a reduction in the mole fraction of Al present in the AlGaAs, e.g., X=0.15. Lattice mismatch is minimized by forming a relatively thin, e.g., 200 .ANG., layer of In.sub.Y Ga.sub.(1-Y) As (Y=0.15) between the doped AlGaAs and an undoped GaAs buffer layer. This MESFET is termed a pseudomorphic HEMT because lattice strain causes the InGaAs layer to distort from a cubic structure to match the lattice constants of each adjacent layer. For further discussion see Swanson, "The pseudomorphic HEMT", Microwaves and RF (March 1987) beginning at page 139.
Although pseudomorphic HEMTs exhibit better high frequency operating characteristics, e.g., 0.43 W/mm at 62 GHz, than other FETs, such performance requires formation of a sharp material transition at the heterostructure interface in order to transfer a large number of electrons into the channel region. In addition, a very thin, e.g., 20-30 .ANG., undoped AlGaAs layer must isolate the heavily doped AlGaAs from the undoped GaAs. Otherwise dopants can easily outdiffuse into the 2DEG region and cause impurity scattering. While stringent requirements such as these can be met in a prototype laboratory environment, i.e., with carefully controlled molecular beam epitaxy techniques, repeatability and uniformity problems must be overcome in order for the device to be manufactured on a monolithic microwave integrated circuit (MMIC) structure or in a volume production environment. Even with metal-organic chemical vapor deposition (MOCVD), it is difficult to abruptly control gas flows in order to repeatably form sharp heterostructure interfaces and thin layers of uniform thickness.
It has been demonstrated that the performance of sub-half-micrometer gate MESFETs can be enhanced by incorporating extremely heavily doped active layers. See Kim et al., "GaAs Power MESFET with 41-Percent Power-Added Efficiency at 35 GHz", I.E.E.E. Electron Device Letters, Vol. 9, No. 2 (February 1988). See also Dambkes et al., "Improved Performance of Micron and Submicron Gate GaAs MESFETS Due to High Electron Concentrations (n=10.sup.18 cm.sup.-3) in the Channel," 1983 GaAs IC Symposium Digest, beginning at page 153.
Schubert et al. have proposed a delta doped MESFET (.delta.FET) composed entirely of GaAs and having a Dirac-delta-function-like profile. See "The Delta-Doped Field-Effect Transistor (.delta.FET)", IEEE Transactions On Electron Devices, Vol. ED-33, No. 5, pp. 625-632, (May 1986). This results in a V-shaped conduction band suitable for confining carrier electrons at a density exceeding 10.sup.18 cm.sup.-3. Theoretically, the impurity layer can be confined to a lattice constant while the electron distribution extends over several lattice constants. In fact, because device operation is in the high field saturation region, formation of such a sharp dopant profile may have a more significant effect on short gate, e.g., 0.25 .mu.m, FET performance than low field mobility characteristics. However, due to high field electron velocity limitations in the GaAs channel region, high frequency performance, e.g., 60 GHz or higher, has not been attainable with pulse-doped GaAs power MESFETs. That is, a gate length on the order of approximately 0.1 .mu.m would be required in order to offset the material limitations of GaAs and thereby provide 60 GHz operation.